WEDNESDAY, March 4 (HealthDay News) -- Many teens today, especially black teens, aren't getting enough of vitamin D, the so-called sunshine vitamin that is essential for cells to function, say researchers from Weill Cornell Medical College in New York City.

Vitamin D deficiency has been linked to many chronic diseases, including heart disease, diabetes, immune system problems and inflammatory diseases.

"There is evidence that the levels of vitamin D we have been using in the past may have been too low," said lead researcher Dr. Sandy Saintonge, a fellow in general preventive medicine at Weill Cornell.

Vitamin D is measured by blood levels of 25-hydroxyvitamin D. Currently, people whose vitamin D level is less than 11 nanograms per milliliter (ng/mL) are classified as deficient, but many experts believe that the minimum level of vitamin D should be at least 20 ng/mL.

Several factors can interfere with the amount of vitamin D the body produces, including diet, sun exposure, use of sunscreen and skin color. Blacks take in less of the sun's rays than whites, causing less vitamin D production, Saintonge noted.

The report was published in the March 3 issue of Pediatrics.

For the study, Saintonge and her colleagues collected data on 2,955 youths, 12 to 19 years old, who participated in the National Health and Nutrition Examination Survey III. The researchers looked specifically at their levels of vitamin D.

Using the latest recommended level of vitamin D, the researchers found that, overall, 14 percent of adolescents in the study were vitamin D deficient. However, black teens were 20 times more likely to be vitamin D deficient than white teens.

Vitamin D deficiency among girls was more than double that of boys. In addition, twice as many obese teens were vitamin D deficient as normal-weight teens, Saintonge said.

She noted that more recent data puts the prevalence of vitamin D deficiency among teens at 40 percent to 50 percent.

Skin color, the prevalence of obesity and diet among black teens combine to make them more likely to be vitamin D deficient, Saintonge said. "It's a multi-factorial problem," she said.

The main dietary source of vitamin D is milk, Saintonge said. But most teens don't drink enough milk, and, to meet the new levels of vitamin D, they would need to drink even more, she said.

Although the exact amount of vitamin D teens need isn't known, it is somewhere between 400 and 1,000 international units (IU) a day, Saintonge said.

Saintonge recommends that teens have their vitamin D level monitored during regular physical examinations to be sure they are getting enough of the nutrient.

But because teenagers are not likely to improve their diets or take vitamin D supplements, more foods need to be fortified with it, Saintonge said. "We need a national fortification strategy," she said. "It is possible that children will still have to take supplements as well."

Dr. Michael F. Holick, director of the Vitamin D Laboratory at Boston University, said that he thinks the study is important, because it will make doctors aware of the problem.

"We know the more skin pigment you have, the less efficient is your ability to make vitamin D in your skin," Holick said. "That's why most African-Americans in the United States are vitamin D deficient."

In addition, he said, obesity prevents vitamin D from being spread throughout the body.

Most experts agree that all teens are getting too little vitamin D, Holick said, adding that he believes they need as much as 2,000 IU a day. "We now recognize that you can take up to 10,000 IUs a day and not worry about any untoward toxicity," he said.

And to combat the problem, Holick agreed that vitamin D should be added to more foods. The Institute of Medicine is expected to recommend dramatically increasing vitamin D levels in 2010, Holick said.

"Then food processors and the food industry will be able to put more vitamin D into the food they fortify now and to put it into more foods," he said.

In addition, Holick said, teenagers need to get more sensible sun exposure.

"Never get a sunburn, wear sunblock on the face -- but don't always slather on sunscreen, because the most efficient way of getting your vitamin D is through your skin," he said. He recommends about 30 minutes of sun exposure a day.